To attain a natural leather-like texture in artificial leather, Patent Document 1 discloses a method in which a knitted/woven fabric is disposed between a screened sheet composed of ultrafine fibers having 0.5 dtex or less and another screened fibrous sheet, the layered sheets are subjected to hydroentanglement, and the thus laminated and entangled material is then filled with an elastic polymer such as polyurethane to give artificial leather having an apparent density of 0.15 g/cm3 or more on the front surface side and an apparent density of less than 0.15 g/cm3 on the back surface side. Moreover, Patent Document 2 discloses a sheet that is made of a nonwoven fabric and a polyurethane binder and has a density gradient in a thickness direction. Furthermore, Patent Documents 3 and 4 disclose a substrate for use in synthetic full-grain leather in which the fineness of the upper layer of the artificial leather substrate is less fine than that of the lower layer. Since the density of a group of fibers in the lower layer is excessively low, the artificial leather of Patent Document 1 does not exhibit a sufficiently rich feel, tautness or resilience, and is not suitable as full-grain leather. In addition, for suede use, this artificial leather has a texture far different from that of natural leather. In Patent Document 2, in the processes after producing a nonwoven fabric, hot pressing and then impregnation with two types of polyurethane are performed to create a density gradient. This method, however, involves a troublesome procedure such as hot pressing, and unlike the natural leather structure having a fineness gradient, qualities such as tautness and resilience that are characteristics of natural leather are not attained. In connection with Patent Documents 3 and 4, two lines of an opening device and a carding machine as well as a plant site covering a large area are necessary to form the substrate having two layers of different line densities. In addition, the barb depth of needles is limited to prevent the fibers on the reverse side from appearing on the front surface, and thus the extent of entanglement is low. Therefore, to provide the strength required for commercialization, the number of times the entanglement is performed, i.e., the number of needles or the number of times the web is required to pass through the line has to be much greater than in a standard method. Thereby the production method is neither reasonable nor economical.    Patent Document 1: JP 55 (1980)-93881 A    Patent Document 2: JP 6 (1994)-280145 A    Patent Document 3: JP 11 (1999)-269774 A    Patent Document 4: JP 2003-13369 A